Environmental Engineering Reference
In-Depth Information
(For formaldehyde oxidation with only two reaction products, the current efficiency
for formic acid formation simply mirrors that for CO
2
formation.) Because of the
very low currents in the low potential region, we have considered only potentials
anodic of 0.6 V in the positive-going scan and anodic of 0.4 V in the negative-
going scan, respectively. In the potential region from 0.6 to 0.7 V in the positive-
going scan, the current efficiency exhibits constant values of about 15% for CO
2
and, correspondingly, about 85% for formic acid formation. With further potential
increase, the current efficiency for CO
2
formation increases and passes through a
double maximum (about 33%) with peaks at 0.75 and 0.9 V, and finally decreases
to about 5% at the positive potential limit of 1.16 V. On the oxidized Pt surface, the
selectivity for formic acid formation is close to 100%. In the negative-going scan,
the current efficiency for CO
2
formation first increases rather slowly, down to poten-
tials around 0.8 V, followed by a faster increase to 25% until 0.45 V, and finally 50% at
0.4 V. The current efficiency for formic acid formation changes correspondingly with
the electrode potential.
For the understanding of the complex shape of the current efficiency for CO
2
formation during formaldehyde oxidation in the positive-going scan, one has to con-
sider contributions from the oxidation of CO
ad
, which was formed during the potential
excursion to more negative potentials, as well. Comparing to the potential of the
formaldehyde adsorbate stripping peak (dashed line in Fig. 13.1b), we attribute the
first peak in the current efficiency for CO
2
formation (dashed line in Fig. 13.4a) to
the oxidation of adsorbed CO
ad
pre-formed at lower potentials. This occurs in addition
to the oxidation of CO
ad
produced by the ongoing formaldehyde decomposition.
Therefore, the first peak can be considered as an additional feature, superimposed
on a broad peak in CO
2
efficiency with its maximum at about 0.8 V. Accordingly,
the second peak in the CO
2
current efficiency at 0.8 V must represent an inherent
maximum in CO
2
selectivity, which is most likely related to the fact that, under
these conditions, surface blocking, either by CO
ad
or by OH
ad
, is minimized.
Apparently, formaldehyde oxidation to CO
2
, either directly or via methylene glycol
oxidation [Batista and Iwasita, 2006], is more site-demanding than oxidation to
formic acid. This interpretation is supported by recent observations showing transient
CO
2
formation upon formaldehyde adsorption/oxidation on a CO
ad
-free Pt surface
even at potentials around 0.3 V, before the reaction is inhibited by accumulation of
a CO adlayer [Heinen et al., 2009]. Finally, at potentials positive of 1.0 V,
formaldehyde oxidation is dominated by selective oxidation to formic acid. The
ability of the oxidized Pt surface to catalyze formaldehyde oxidation to formic acid
under these conditions, but not methanol oxidation, cannot be explained by a more
facile decomposition of formaldehyde to CO
ad
, compared with methanol, since, in
that case, CO
2
should be the dominant oxidation product in formaldehyde oxidation,
rather than formic acid. Hence, oxidation of the Pt surface prevents formaldehyde
decomposition. Similar observations were made also for acetaldehyde, where
oxidation at high potentials is facile and results exclusively in acetic acid formation
[Wang et al., 2006]. Apparently, the oxidative dissociation of the aldehyde C - H
bond is possible on an oxidized Pt catalyst at potentials E . 0.9 V. In the reverse,
negative-going scan, accumulation of CO
ad
is inhibited at potentials anodic of the
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